1. Technical Field
The invention is directed to a process for fabricating integrated circuit devices and in particular utilizing a measuring technique in conjunction with such processes.
2. Art Background
The move towards smaller design rules for the fabrication of integrated circuits is motivated by the desire to place a greater number of devices on a chip. For advanced device structures, both the film thickness of layers used to form these structures and the structure width must decrease as the number of devices on a chip increase. The presence of these smaller features, which are smaller in width and in thickness, has increased the complexity of fabricating integrated circuits. It is especially difficult to control the etching process (for example) of an integrated circuit when rapidly removing multiple layers of different materials from over a thin layer which must not be substantially affected. Typically, such processes require complex changes in processing conditions that must be made quickly.
An ellipsometer is frequently used to determine the characteristics of blanket films in the context of device fabrication. A beam of light is directed onto the surface of the film. The ellipsometer measures the light reflected from the film. From the reflected light the ellipsometer determines the angles DELTA (.DELTA.) and PSI (.PSI.), which are defined as the change in phase of the light and the arc tangent of the factor by which the amplitude ratio of the incident and reflected light in the direction parallel to the plane of incident light changes, respectively. These quantities are then used to determine the optical characteristics such as the index of refraction, the film thickness, and the extinction coefficient of unpatterned, uniform films.
The use of an ellipsometer to derive .DELTA. and .PSI. coordinates of a polarized light beam reflected from a work piece to monitor the thickness of a film deposited on a substrate is described in U.S. Pat. No. 5,131,752 to Yu et al. Yu et al. first calculate the endpoint, which is the point when a film of the desired thickness has been deposited, from the .DELTA. and .PSI. values of a film of the desired thickness. Yu et al. calculate end point values of .DELTA. and .PSI. from the known angle of incidence, the wavelength of the light source, the desired final film thickness, and the optical constants of the substrate and film at the process temperature. Therefore, the Yu et al. method is limited to controlling a process in which the precise endpoint is known.
In the context of device fabrication using masks or over topography, the precise endpoint of any process is difficult to determine due to the attendant irregularity of the surface. Masks as used herein are structures which are used to introduce a pattern onto or into a film or films overlying a substrate. Topography as used herein are surface irregularities or structures underlying films on a substrate. The optical path of an incident beam of light from an ellipsometer is affected by the presence of a mask or topography on a wafer, because the composition and/or configuration of the surface is irregular. Wafer, as used herein, is a substrate with films thereon. These irregularities cause the light reflected from the surface to be different than light reflected from uniform, blanket films. The mask or topography also affects the planarization state of light and therefore creates interference which adversely affects the quality of the signal that is reflected from the surface.
Henck, Steven A., et al., "In situ spectral ellipsometry for real-time thickness measurement: Etching multilyer stacks," J. Vac. Sci. Technol. A., 11(4): 1179 (July/Aug. 1993) propose using an ellipsometer to monitor the film thickness in a large unpatterned region at the center of the wafer during plasma etching. The ellipsometer is first calibrated using known techniques. Then the ellipsometric parameters, .DELTA. and .PSI., are determined from the thickness and the dielectric function of the film material. As the etch proceeds, the ellipsometer continues to transmit a beam of light toward the film and to measure the properties of the reflected light to obtain a useful signal. The changes in the reflected light indicate the decreasing thickness of the top layer of the film and the increasing proximity of the interface between the top layer and the layer underlying the top layer. Thus, during etching, an ellipsometer is used to determine how much of the top layer has been removed, which enables the etching to be terminated with a known film thickness remaining.
The technique described by Henck et al. requires an unpatterned topography-free area (or test pad) on the wafer in which to make the required measurements. An unpatterned, topography-free area on a device is undesirable, because it sacrifices that portion of the device real estate. Also, since an unpatterned, topography-free test pad requires different masking, etching, and deposition steps than patterned topography-containing areas, it can also complicate the lithographic process. Therefore, manufacturing costs will increase if a significant portion of the wafer has to be sacrificed to provide an area on which to perform the technique described by Henck et al.
Haverlag, M., et al., "In situ ellipsometry and reflectometry during etching of patterned surfaces: Experiments and simulations," J. Vac. Sci. Technol. B, 10(6):2412 (Nov./Dec. 1992) describe the use of ellipsometry at a single wavelength (632 nm) for the end point detection of a plasma etching process. Haverlag et al. conclude that such a technique cannot be used over typical patterned wafers because the end point was not detected when the incident ellipsometric light beam was aimed onto the surface in a direction that was perpendicular to the direction of lines formed by a mask over the film. Haverlag et al. observed that reflections of the light on the mask sidewalls inhibit the light from reaching the ellipsometric detector and concluded that an ellipsometric technique for plasma etch end point detection could only be used on patterned wafers with low, e.g., less than 0.3, aspect ratios.
However, in many applications for fabricating real devices, the aspect ratios of masks or topography are typically greater than 1.0. Also, as design rules decrease, more severe topography and higher aspect ratios are expected. Therefore, an ellipsometric technique for process control that can be used in processes for fabricating small design rule devices with topography over essentially all of the entire surface of the wafer is desired.